Lantibiotics are post-translationally modified peptide antibiotics. Nisin, its best known member, has been used for decades in the food industry without significant development of resistance. Other lantibiotics are under investigation or in clinical trials for the treatment of cystic fibrosis and asthma, the elimination of tooth-decay causing bacteria, and for the treatment of multi-drug resistant bacteria. In this grant application we seek to extend our past success towards the use of the biosynthetic machinery to make analogs and to better understand their mechanism of substrate recognition and catalysis. Our goals are: 1. Understand the enzymes involved in crosslink formation 2. Engineer efficient processes to manipulate the structures of posttranslationally modified peptide natural products 3. Investigate new posttranslationally modified peptide natural products.